1. Field of the Invention
The present invention relates to a method of fabricating a semiconductor device, and more particularly to a method of fabricating an ETOX flash memory.
2. Description of the Related Art
ETOX flash memory is a type of erasable programmable read only memory (EPROM) that also incorporates a thin tunnel oxide structure. It is therefore called "ETOX" (EPROM with Tunnel Oxide) flash.
According to a method of fabricating ETOX flash memory cell in the prior art, a plurality of parallel field oxide lines are formed in the substrate to serve as a device isolation structure. Stacked word lines having a tunneling oxide layer, a floating gate, an ONO (oxide-nitride-oxide) layer and a control gate are then formed on the substrate, wherein the stacked word lines are parallel to each other and perpendicular to the field oxide lines. A buried source line and drain region are formed on each side of the stacked word line by implanting ions in the substrate. A contact plug connected to a bit line is formed on the drain region. The buried source line is perpendicular to the field oxide line, and the bit line is parallel to the field oxide line.
Since the buried source line in the prior art crosses the field oxide line, the buried source line is curved and therefore has a varied distribution because of the field oxide line. However, the field oxide line limits the shrinkage of the ETOX flash memory, as the integration of the integrated circuit increases. Hence, shallow trench isolation (STI) lines substituted for the field oxide lines are employed to solve the problem. But, as shown in FIG. 1, while fabricating the buried source line 16, ions 14 are generally implanted in a direction perpendicular to the silicon substrate 10, an distribution of the doped ions follows the profile of the STI lines 12. A very thin ion-doped layer 18 is formed next to the sidewalls of the STI lines 12. Consequently, discontinuity and high resistance may occur somewhere along the source line 16, and operation speed of the ETOX flash memory is therefore decreased..